1. Technical Field
The present invention relates to a group-III nitride semiconductor freestanding substrate such as a GaN freestanding substrate, and a manufacturing method of the same, and particularly relates to the group-III nitride semiconductor freestanding substrate capable of reducing warpage of the freestanding substrate.
2. Description of Related Art
A group-III nitride semiconductor material has a sufficiently wide energy band gap, and an interband transition is a direct transition, and therefore it has been actively developed so as to be applied to a short wavelength light emitting element. In addition, a saturated electron drift velocity is fast, and two dimensional carrier gas by hetero junction can be used. Therefore application to an electron element is also expected.
As a method of obtaining the group-III nitride semiconductor freestanding substrate with relatively less crystal defects, it is possible to use a method such as that a heterogeneous substrate (foreign substrate) like a sapphire substrate is set as a base substrate, then a GaN crystal is epitaxially-grown thick on this base substrate by using a hydride vapor phase epitaxy (HVPE method) in which a crystal growing speed is fast, and after end of the growth, the base substrate is removed by a certain method, to thereby use the remained GaN crystal layer as a GaN freestanding substrate.
For example, a mask having an opening part is formed on the base substrate like a sapphire substrate, and by using a technique of obtaining a GaN layer with few dislocations by making the GaN layer lateral-grown from the opening part, namely, a so-called ELO (Epitaxial Lateral Overgrowth) technique, the GaN layer is formed on the sapphire substrate, and thereafter the sapphire substrate is removed by etching, etc, to thereby obtain the GaN freestanding substrate.
Also, a DEEP (Dislocation Elimination by the Epi-growth with Inverted-Pyramidal Pits) method is a method of removing a GaAs substrate by a method such as etching, after the GaN layer is grown by using the mask such as silicon nitride patterned on the GaAs substrate.
Further, according to a VAS (Void-Assisted Separation) method, the GaN layer is grown, with a void layer interposed between the sapphire substrate and the GaN layer, and the GaN layer is separated at a boundary of the void layer after end of the growth (for example, see patent document 1).
As the GaN substrate used in fabricating the light emitting element, a conductive substrate is normally used for the purpose of fabricating a light emitting device having an upper/lower electrode structure in which an electrode (upper electrode) is provided in an upper part of epitaxial layers formed on the surface of the GaN substrate, and also an electrode (lower electrode) is provided on a rear surface of the GaN substrate. n-type is frequently used in a conductive type of the GaN substrate. This is because p-type conductivity can not be obtained if thermal processing and activation by electron beam irradiation are not performed, after GaN is crystal-grown, with Mg doped thereinto. Therefore, n-type is used as the conductive type of the GaN substrate, and a device structure is designed in such a manner that an outermost layer of epitaxial layers structure is a p-type layer.
When the n-type GaN crystal is grown by a metal-organic vapor phase epitaxy (MOVPE method), normally a method of doping Si, with silane (SiH4) and disilane (Si2H6) used as doping gas, is employed.
However, in the HVPE method suitable for high-speed growth, silane and disilane can not be used as doping gas. This is because since the HVPE method is a so-called hot wall type crystal growth system in which source gas is brought into contact with a reactor wall heated at high temperature, silane and disilane are decomposed before they reach the substrate and are not taken into crystal effectively.
Therefore, patent document 2 proposes a technique of doping Si by using dichlorosilane (SiH2Cl2) as doping gas, when the n-type nitride semiconductor crystal is grown by the HVPE method. Also, patent document 3 proposes a technique of manufacturing the GaN substrate with n-type conductivity by using oxygen as dopant.    (Patent document 1)    Japanese Patent Laid-Open Publication No. 2003-178984    (Patent document 2)    Japanese Patent Laid-Open Publication No. 2000-91234    (Patent document 3)    Japanese Patent Laid-Open Publication No. 2000-44400
However, as described above, since the GaN crystal is grown on the heterogeneous substrate, difference in coefficient of linear expansion is generated between the heterogeneous substrate and an initial GaN crystal layer at initial time of growth and difference of defect density is generated in a thickness direction of the GaN layer, thereby generating a stress in the GaN crystal, resulting in a warpage generated in the as-grown GaN substrate, and in some cases, the substrate can not withstand the stress and crack is generated. Further, since the warpageed GaN substrate is flattened, variation of off-angles occurs in the surface of the substrate. By these problems, conventionally, a device manufacture by using the GaN substrate is difficult.